Conventional signal reception systems are degraded in signal-to-noise ratio performance because of the presence of undesired signals that enter the system by the main lobe or the side lobes of an antenna. Such signals may consist of hostile, deliberate interference or non-hostile unintentional interference. Natural noise sources almost always exist and in certain systems, received signals are reflected from xe2x80x9cclutterxe2x80x9d in the antenna field. As communication traffic by wireless and other media increases substantially, poor siting conditions and multipath beam effects add to a worsening interference environment. Newer, unlicensed radio stations using xe2x80x9cweak signalxe2x80x9d communication are particularly vulnerable to interference from other stations operating on the same frequencies. Interference with desired signals by signals on the same or very near frequency is often referred to as xe2x80x9cjammingxe2x80x9d even in cases which are non-deliberate. To insure the quality of service expectations of users of such systems, it is important that they be able to transmit and receive traffic without interference from jamming signals.
In the book by Robert A. Monzingo and Thomas W. Miller, entitled Introduction to Adaptive Arrays, John Wiley and Sons, New York, 1980, the authors describe adaptive antenna arrays that have been used as a solution to problems of signal detection in severe interference environments.
U.S. Pat. No. 5,724,666 was issued on Mar. 3, 1998 to Paul W. Dent and is entitled Polarization Diversity Phase Array Cellular Base Station and Associated Methods. The Patent describes a base station which includes a first and second antenna array receiving first and second rotational polarizations and a polarization diversity receiver connected to the first and second arrays.
In his U.S. Pat. No. 5,691,727, Adaptive Polarization Diversity System, issued on Nov. 25, 1997, Baruch Cyzs describes an adaptive polarization diversity system for radio link communication. Hans Mahler was issued U.S. Pat. No. 5,963,874 for a Radio Station Arranged for Space Diversity and Polarization Diversity Reception, Oct. 5, 1999. The patent describes antennas arranged for reception of signals with different polarization directions for achieving polarization diversity reception.
U.S. Pat. No. 4,298,873 is entitled Adaptive Steerable Null Antenna Processor and was issued on Nov. 3, 1981 to Eugene L. Roberts. The invention described in this Patent is an adaptive array system which steers a plurality of antennas to direct a null toward an undesirable source of interference, in order to reduce noise and enhance detection of signals.
Eugene J. Bruckert, et al. were granted U.S. Pat. No. 4,584,713: Signal Quality Steered Diversity on Apr. 22, 1986. The Patent discloses a circuit and technique for directing an adaptive antenna system. U.S. Pat. No. 5,649,287, issued Jul. 15, 1997, to Ulf Gxc3x6ran Forssxc3xa9n, et al., entitled: Orthogonalizing Methods for Antenna Pattern Nullfilling, discloses a method of broadcasting in a cellular communication system where common information creates orthogonal signals.
Eugene L. Roberts was granted U.S. Pat. No. 5,107,273 entitled Adaptive Steerable Null Antenna Processor With Null Indicator in April, 1992. The disclosure describes an adaptive array system which steers a plurality of antennas to direct a null toward an undesired source of interference to reduce noise and enhance detection of desired signals.
U.S. Pat. No. 4,670,885 issued to Steen A. Parl, et al. and entitled Spread Spectrum Adaptive Antenna Interference Canceller, describes an adaptive power equalizer circuit for use in a spread spectrum receiver system.
A Sidelobe Canceller with Adaptive Antenna Subarraying Using a Weighted Butler Matrix is the subject of U.S. Pat. No. 4,596,986 which was issued to Grealie A. Andrews, et al. on Jun. 24, 1986. An apparatus for nulling signals from unwanted interference sources is disclosed.
U.S. Pat. No. 4,152,702 is entitled Adaptive Antenna Lobing on Spread Spectrum Signals at Negative Signal-to-Noise Ratio. It discloses a communications system including a multiple antenna array with null steering apparatus.
What is currently needed is a method and system for discriminating against unwanted, interfering signals from a fixed source, while maintaining the antenna gain in respect of receiving desired signals. The invention provides a solution to this and other problems.
In one aspect of the present invention, a method is provided for receiving desired signals having a desired circular polarization in the presence of interfering signals, comprising receiving the desired signals and interfering signals in a first antenna having the desired circular polarization to form a first output signal, receiving the interfering signals in a second antenna having an opposite circular polarization to the desired circular polarization to form a second output signal, and combining the first and second output signals to minimize the interfering signals.
In a further aspect of the invention, the output signals are weighted prior to linearly combining the output signals to minimize the interfering signals. Weighting the output signals may include selecting a first gain for the first antenna and a second gain for the second antenna, deriving complex weights for each antenna corresponding to the selected first gain and second gain, applying the complex weights to the respective output signals, and linearly combining the weighted output signals to minimize the interfering signals.
In another aspect of the invention, receiving the interfering signals may comprise receiving the interfering signals in the first and second antennas over a period of time during which no desired signals are transmitted, and selecting the first and second gains comprises selecting the first and second gains to minimize one or more of the received interfering signals. Receiving the desired signals may alternatively comprise measuring a performance value for the desired signals, and selecting the first gain comprises selecting the first gain to maximize the performance value for one or more of the received desired signals.